The goal of this project is to increase our understanding of the development of the hair cells and neurons of the inner ear, and of how failure of normal developmental processes leads to hearing loss and deafness. In the current period of funding we obtained evidence that hair cells depend for their differentiation and survival upon a POU-domain transcription factor, Brn-3.1, also known as POU4f3, which is associated with inherited deafness DFNA 15 in humans. Deletion of this factor in mice results in failure of hair cell precursors to produce both morphological and molecular features characteristic of mature inner ear sensory cells, followed by the death of many of these cells. This failure of hair cell development results in profound deafness and lack of vestibular function. A second POU-domain factor, Brn-3.0, is required for the normal development of spiral ganglion and primary vestibular neurons. In this application, we propose to further investigate the roles of Brn-3.1 and Brn-3.0 in hair cell and inner ear neuronal development, respectively. To accomplish this we will first explore the control of Brn-3.1 expression in hair cells. Second, we will further characterize the consequences of deletion of Brn-3.1 and Brn-3.0 on the inner ear. In order to accomplish this, we will identify transcripts that are differentially expressed in hair cells due to the deletion of Brn-3.1, investigate the causes of hair cell death in the absence of Brn-3.1, and study mechanisms leading to spiral ganglion defects in the absence of Brn-3.0. Third, we will investigate the importance of different regions of the Brn-3.1 molecule in determining hair cell differentiation and survival, by replacing Brn-3.1 with Brn-3.1/3.0 chimeras. Information about the regulation of the Brn-3.1 expression will help to determine how the production of hair cell-specific proteins is controlled. It could also help identify factors that are upstream of Brn-3.1, and may illuminate the processes that lead cells to adopt the hair cell phenotype. Identification of factors that are downstream from Brn-3.1, and of Brn-3.1 domains that are critical for hair cell development and survival, are critical steps in understanding the role of this gene in inherited deafness.